Heralded single-photon sources from parametric down-conversion

Short answer: Heralded single-photon sources use spontaneous parametric down-conversion, where a nonlinear crystal emits photon pairs and detecting one photon announces the other. A 2008 Physical Review Letters paper reports pure heralded photons from spectrally engineered sources with a 94.4% interference visibility, and a 2015 Physical Review A paper reports that filtering can still give pure photons with high heralding efficiency. A later New Journal of Physics paper frames the tradeoff between rate, purity and multi-photon noise. All are laboratory studies.

How the source works

A New Journal of Physics paper on the topic describes these as heralded single photon sources. They produce photons in pairs, so detecting one photon heralds the presence of the other. The authors say sources based on nonlinear crystals are still the standard in applications, being compact, able to produce photon states of high spectral purity and indistinguishability, and easy to package in integrated, room-temperature devices.

2008: pure photons without filters

The Physical Review Letters abstract, by Mosley and coauthors, presents an experimental demonstration of heralded single photons in pure quantum states from a down-conversion source. By controlling the modal structure of the pair emission, they generate pairs in factorable states, which removes the need for spectral filters in multiple-source interference schemes. Indistinguishable heralded photons were produced in two independent spectrally engineered sources, and Hong-Ou-Mandel interference between them was observed without spectral filters. The published abstract gives a measured visibility of 94.4%, which it says sets a minimum bound on the mean photon purity. The arXiv version of the abstract phrases this as a raw visibility of 94.4% with purity measured to be over 95%. The two wordings differ, so this post quotes the published figure only.

2015: filters and heralding efficiency

The Physical Review A abstract, by Florez and coauthors, studies single photons from spontaneous parametric down-conversion when both the spectral and spatial degrees of freedom are controlled by filters. The authors report that it is possible to obtain pure heralded single photons with high heralding efficiency despite the use of filters. They also report an asymmetry in the single-photon properties of type-II sources that depends on whether the signal or the idler photon is used as the heralding one.

The rate and noise tradeoff

The New Journal of Physics introduction states that applications need higher rates of single-photon generation while keeping the photons spectrally pure and keeping multi-photon noise low. In down-conversion sources, the authors say these requirements conflict. Spectral filtering to raise purity lowers the generation rate, and raising the pump power raises the multi-photon noise. The paper goes on to present a scheme to address this, but only its opening text was readable here, so the scheme is not described.

Limits

These sources are probabilistic, which is why the pump level and the heralding choice matter. Each paper reports figures from its own setup and settings, and the abstracts do not compare the approaches under common conditions, so no ranking is implied.

FAQ

What does heralded mean?

Detecting one photon of a pair announces that its partner is present.

Why does spectral purity matter?

Pure, indistinguishable photons are needed for multi-photon interference experiments, as in the 2008 paper.

Can filtering still give pure photons?

The 2015 paper reports pure heralded photons with high heralding efficiency despite filters.

What is the main tradeoff?

Filtering raises purity but lowers the rate, and higher pump power raises multi-photon noise, per the New Journal of Physics paper.

What visibility did the 2008 experiment reach?

94.4% in Hong-Ou-Mandel interference between two independent sources, per the published abstract.